In 1971, president Richard Nixon publicly declared war on drugs, naming drug abuse as "public enemy number one in the United States". Over four decades and countless public education campaigns later, it's safe to say that the war has not been won -- not by a longshot.

The silver lining in that cloud is that Americans are much smarter about drugs today than they once were. They realize that the dream of eradicating illegal drugs altogether is impossible. And so, like pragmatists, most have focused their attention on limiting the spread of harder drugs like crack and heroin -- dangerous, cheap drugs that often kill users and destroy communities.

In the process, attitudes toward softer drugs like marijuana have, well, softened. Many states now allow pot for medicinal use, and during the November 2012 elections, voters in Colorado and Washington state legalized recreational use, too.

To many Americans, that's fine. Pot is often believed to be far less dangerous than alcohol, and several studies (plus at least one CNN segment) support that claim.

But now there's evidence for the other side of the argument, and it shows that marijuana has played a role in a growing number of automobile accidents.

NEW RESEARCH

The evidence comes by way of a study from researchers at Columbia University's Mailman School of Public Health. The team looked at auto fatality statistics from 1999 to 2010 pulled from six states: California, Hawaii, Illinois, New Hampshire, Rhode Island, and West Virginia. Those states were chosen because they regularly run toxicology tests on all drivers involved in fatal accidents. All told, the study looked at more than 23,500 roadway deaths that occurred within one hour of a collision.

What the researchers found was that alcohol was a contributing factor in about 40 percent of those accidents -- a figure that remained constant throughout the period studied. In other words, education programs had little effect on the rate of accidents involving alcohol: no matter what programs were in place, alcohol had the same effect on fatality statistics in 1999 as it did in 2010.

Drugged driving, however, became increasingly common over the period. In 1999, drugs were found in the bloodstreams of 16 percent of drivers involved in fatal accidents. By 2010, that figure had risen to 28 percent. According to co-author Dr. Guohua Li, "If this trend continues, in five or six years non-alcohol drugs will overtake alcohol to become the most common substance involved in deaths related to impaired driving."

It should be noted that the summary report doesn't clearly distinguish between legal drugs like painkillers and illegal drugs like cocaine and marijuana. However, one thing is clear: of all the drugs found in drivers' blood systems, pot was the most common. In 1999, it was found in the bloodstreams of just four percent of drivers, but by 2010, that number reached 12 percent.

MORE STUDY NEEDED

Although advocacy groups like Mothers Against Drunk Driving were quick to use the study's findings to support their own campaigns against impaired driving, the study's authors admit that the research is imperfect, leaving many questions unanswered. For example:

1. Is pot really to blame for these accidents? Unlike alcohol, marijuana can remain in the bloodstream for weeks after it has been consumed -- possibly affecting driving skills, possibly not. (Consider this: CNN found that Washington's limit of five nanograms of THC per milliliter of blood wasn't nearly high enough to negatively impact driving.) The study's authors caution that their findings don't indicate that the drivers were impaired from pot at the time of their accident, only that they had used pot sometime in the recent past.